1. Field of the Invention
This invention relates to a GaInP epitaxial stacking structure, and more specifically to a GaInP epitaxial stacking structure for FETs and a fabrication method thereof, which has electron-supply layers and spacer layers which give high-mobility characteristics, and a high-mobility field effect transistor using this structure.
2. Description of the Prior Art:
Schottky junction-type field effect transistors (known as MESFETs) which operate in the microwave region or millimeter wave region include GaInP high electron mobility transistors (known as TEGFETs, MODFETs and the like) which utilize mixed crystals of gallium-indium phosphide (GaAIn1xe2x88x92AP: 0xe2x89xa6Axe2x89xa61) (see IEEE Trans. Electron Devices, Vol. 37, No. 10 (1990), pp. 2141-2147). GaInP MODFETs can be used as low-noise MESFETs for signal amplification in the microwave region (see IEEE Trans. Electron Devices, Vol. 46, No. 1 (1999), pp. 48-54) and as power MESFETs for transmission applications (see IEEE Trans. Electron Devices, Vol. 44, No. 9 (1997), pp. 1341-1348).
FIG. 1 is a schematic diagram of the cross-sectional structure of a conventional GaInP TEGFET. The substrate 10 used is made of semi-insulating gallium arsenide (chemical formula: GaAs) with a {001} crystal plane as its primary plane. Upon the substrate 10 is deposited a buffer layer 11 consisting of a high-resistance Group III-V compound semiconductor layer. Upon the buffer layer 11 is deposited an electron transporting layer (channel layer) 12 consisting of n-type mixed crystals of gallium-indium arsenide (GaZIn1xe2x88x92ZAs: 0 less than Zxe2x89xa61). A spacer layer may be deposited upon the channel layer 12, but particularly in power TEGFETs for transmission applications, an electron-supply layer 13 consisting of mixed crystals of gallium-indium phosphide (GaYIn1xe2x88x92YP: 0 less than Yxe2x89xa61) is deposited without an interposed spacer layer. The carrier (electron) density of the electron-supply layer 13 is adjusted by the intentional addition (doping) of silicon (Si) or other n-type impurities which are not readily diffused. Upon the electron-supply layer 13, a contact layer 14 consisting of n-type GaAs or the like is typically provided in order to form the low-contact resistance source electrode 15 and drain electrode 16. In addition, between the source and drain electrodes 15, 16, the contact layer 14 is partially removed to expose a recess structure, and a Schottky junction-type gate electrode 17 is provided, thereby constituting a TEGFET.
The various constituent layers 11-14 which constitute the GaInP epitaxial stacking structure 1A for MODFET application illustrated in FIG. 1, because of their ease of film formation, are conventionally formed by the metal-organic chemical vapor deposition (MOCVD) method (see ibid IEEE Trans. Electron Devices, Vol. 44 (1997)). Among these constituent layers, the electron-supply layer 13 is a functional layer for supplying electrons formed to accumulate as a two-dimensional electron gas (TEG) in the vicinity of the junction interface 12a of the channel layer 12. The electron-supply layer 13 is conventionally formed of gallium-indium phosphide (GaYIn1xe2x88x92YP: 0 less than Yxe2x89xa61) doped with silicon (the symbol of element: Si) or other n-type impurities which are not readily diffused (see ibid IEEE Trans. Electron Devices, Vol. 44 (1997)). The carrier density (units: cmxe2x88x923) of the electron-supply layer 13 is commonly made 1-3xc3x971018 cmxe2x88x923 or 2xc3x971018 cmxe2x88x923 in particular. The thickness of the layer is typically set within the range 10 nm to 40 nm. In addition, in a GaInP TEGFET, the n-type electron-supply layer is normally constituted from GaYIn1xe2x88x92YP (0 less than Yxe2x89xa61) layers wherein the gallium composition ratio (=Y) is fixed in the layer thickness direction.
In addition, in the structure wherein a spacer layer is deposited upon the channel layer 12, in order to prevent the two-dimensional electron gas from being disturbed due to ionization scattering from the channel layer 12, the spacer layer is a functional layer provided for the spatial isolation of the channel layer 12 and electron-supply layer 13 (see xe2x80x9cPhysics and Applications of Semiconductor Superlattices,xe2x80x9d Physical Society of Japan, ed. (published by Baifukan, Sep. 30, 1986, first edition, fourth printing), pp. 236-240). In a GaInP TEGFET, the spacer layer is typically constituted from undoped GaXIn1xe2x88x92XP (0 less than Xxe2x89xa61) (see ibid IEEE Trans. Electron Devices, Vol. 44 (1997)). Regardless of the case of GaInP TEGFET, spacer layers are constituted from high-purity undoped layers with a low total amount of impurities, and their layer thickness is typically in the range from 2 nanometers (nm) to 10 nm (see ibid xe2x80x9cPhysics and Applications of Semiconductor Superlattices,xe2x80x9d pp. 18-20).
For example, in a low-noise GaInP TEGFET, the noise-figure (NF) and other major properties vary depending on the electron mobility, so the higher the electron mobility, the lower the NF conveniently becomes. For this reason, in order to cause the electrons supplied from the n-type electron-supply layer 13 to accumulate as a two-dimensional electron gas in the interior regions of the GaZIn1xe2x88x92ZAs (0 less than Zxe2x89xa61) in the vicinity of the junction interface with the spacer layer consisting of undoped GaXIn1xe2x88x92XP (0 less than Xxe2x89xa61), the composition at the junction interface between the channel layer 12 and the spacer layer must change abruptly and exhibit high electron mobility.
In addition, the formation of a buffer layer is typically performed by vapor deposition without varying the starting material species of gallium (element symbol: Ga). Since the admixture of carbon (element symbol: C) acceptors that electrically compensate residual donor components represented by silicon occurs readily, and a high-resistance GaAs layer or AlLGa1xe2x88x92LAS layer is easily obtained in the undoped state (see J. Crystal Growth, 55 (1981), pp. 255-262), trimethyl gallium (chemical formula: (CH3)3Ga) is used as the gallium (Ga) source (see J. Crystal Growth, 55 (1981), pp. 246-254, ibd, pp. 255-262, and PCT application publication No. 10-504685).
In a GaInP TEGFET for low-noise amplification, the noise-figure (NF) and other major properties vary depending on the two-dimensional electron mobility (units: cm2/Nxc2x7s), so the higher the electron mobility (cm2/Nxc2x7s), the lower the NF becomes. For this reason, in a low-noise TEOFET, the electron-supply layer which takes the role of supplying electrons must be constituted from GaYIn1xe2x88x92YP (0 less than Yxe2x89xa61) which can exhibit a high electron mobility. On the other hand, in a power TEGFET, from the standpoint of causing it to operate with a relatively large source-drain current flowing, a large sheet carrier density (units: cmxe2x88x922) is required together with the electron mobility. Therefore, electron-supply layer for power TEGFET applications must be constituted from a GaYIn1xe2x88x92YP (0 less than Yxe2x89xa61) layer that exhibits a high sheet carrier density.
However, in the conventional electron-supply layer consisting of GaYIn1xe2x88x92YP wherein the gallium composition ratio (=Y) or indium composition ratio (=1xe2x88x92Y) is roughly constant, at a relatively high sheet carrier density, there is a disadvantage in that a high electron mobility cannot be manifested stably. For this reason, in low-noise GaInP TEGFETs for example, a large transconductance (gm) is not obtained, thus obstructing the stable supply of low-noise GaInP TBGFBTs with a superior low noise-figure (NF).
A first object of the present invention is to provide a GaInP epitaxial stacking structure containing a GaYIn1xe2x88x92YP (0 less than Yxe2x89xa61) electron-supply layer and fabrication method thereof for stably manifesting a high electron mobility in excess of 5000 cm2/Vxc2x7s at room temperature and at a relatively high sheet carrier density of 1.5xc3x971012 cm2 or greater and 2.0xc3x971012 cmxe2x88x922 or less. With this structure, low-noise GaInP high electron mobility transistors with superior transconductance properties and power GaInP TEGFETs with superior power transformation efficiency due to their high source-drain current can be provided.
In addition, in a structure wherein a spacer layer is provided between the channel layer and electron-supply layer, if a GaXIn1xe2x88x92XP spacer layer wherein the indium composition ratio (=1xe2x88x92X) is roughly constant is provided joined to the GaZIn1xe2x88x92ZAs (0 less than Z less than 1) channel layer 12, mutual diffusion occurs between phosphorus (element symbol: P) and arsenic (element symbol: As) in the vicinity of the junction interface 12a, so a problem occurs in that the steep change in composition at the junction interface 12a is worsened.
If the steepness of change in composition at the junction interface 12a is not achieved, a two-dimensional electron gas does not efficiently accumulate in the interior regions of the GaZIn1xe2x88x92ZAs channel layer 12, and the electron mobility drops. The electron mobility particularly affects the transconductance (gm) of GaInP TEGFETs for low-noise amplification, and influences the noise-figure (NF) even more. At a low electron mobility, a high gm is not obtained and therefore, a GaInP TEGFET with a low NF is not obtained.
In addition, it was conventionally common for the spacer layer to be constituted from an undoped GaXIn1xe2x88x92XP (0 less than Xxe2x89xa61) layer wherein the indium composition ratio is constant. However, the carrier density in the undoped state is roughly 1xc3x971016 cmxe2x88x923 at the lowest. Since the two-dimensional electron gas accumulates more efficiently by lowering the carrier density of the spacer layer, in order for a high electron mobility to be manifested, the spacer layer must be constituted from a GaXIn1xe2x88x92XP (0 less than Xxe2x89xa61) layer with an even lower carrier density.
Thus, a second object of the present invention is to provide an epitaxial stacking structure comprising a spacer layer made of GaXIn1xe2x88x92XP (0 less than Xxe2x89xa61) which can stably manifest an even higher electron mobility and has a low carrier density. With this structure, it is possible to provide a GaInP epitaxial stacking structure with excellent transconductance.
Irrespective of GaInP TEGFETs, the transconductance (gm) and pinch-off characteristics of high electron mobility field effect transistors are known to fluctuate depending on the quality of the buffer layer. For example, in the normal AlGaAs/GaAs lattice-matched TEGFETs and AlGaAs/GaInAs strained-lattice TEGFETs, a high gm and good pinch-off characteristics are obtained, and the buffer layer is formed as a high-resistance layer with a low leakage current.
On the other hand, as described above, in a GaInP TEGFET comprising an electron-supply layer consisting of GaYIn1xe2x88x92YP which is one type of a phosphorus-(element symbol: P) containing Group III-V compound semiconductor, simply making the buffer layer a high-resistance layer has conventionally had the problem wherein a homogenous gm and pinch-off voltage cannot be stably obtained. The present inventors discovered that this instability of properties derives from heterogeneity in the indium composition ratio (=1xe2x88x92Y) of the GaYIn1xe2x88x92YP electron-supply layer due to differences in the gallium (Ga) source utilized in the formation of the buffer layer of a superlattice structure that uses AlGaAs and GaAs in particular as constituent layers.
In addition, in the buffer layers consisting of the conventional constitution such as AlGaAs/GaAs superlattice-structure buffer layers, there are problems regarding the DC properties (static properties) of the transistor in that fluctuation in the source-drain current value under illumination (so-called xe2x80x9cphotoresponsibilityxe2x80x9d) (see G. J. Ree, ed., Semi-Insulating III-V Materials, (Shiva Pub. Ltd. (Kent, UK, 1980), pp. 349-352) and xe2x80x9chysteresisxe2x80x9d of the source-drain current (see Makoto Kikuchi, Yasuhiro Tarui, eds., xe2x80x9cIllustrated Semiconductor Dictionary,xe2x80x9d (Nikkan Kogyo Shimbunsha, Jan. 25, 1978), p. 238) and xe2x80x9ckinksxe2x80x9d easily occur (JP-A-10-247727 and JP-A-10-335350).
Therefore, a third object of the present invention is to provide an epitaxial stacking structure comprising a buffer layer for forming a GaYIn1xe2x88x92YP (0 less than Yxe2x89xa61) electron-supply layer that has high resistance suitable for reducing the leakage current and that has a homogeneous indium composition.
In a GaInP TEGFET, the spacer layer is constituted from GaXIn1xe2x88x92xP (0 less than Xxe2x89xa61) which is an indium-containing Group III-V compound semiconductor, and moreover it is constituted as a thin film. The conventional MOCVD technology has a problem in that thin-film spacer layers with a homogenous indium composition ratio (=1xe2x88x92X) cannot be stably obtained.
For this reason, conventional GaInP high electron mobility field effect transistors which use as the spacer layer a GaXIn1xe2x88x92XP (0 less than Xxe2x89xa61) layer wherein the indium composition ratio is not sufficiently homogenous cannot maintain a homogenous band offset with the channel layer due to xe2x80x9cfluctuationxe2x80x9d in the indium composition ratio within the spacer layer, and for this reason, achieving a homogenous transconductance (gm and pinch-off voltage was difficult.
Therefore, a fourth object of the present invention is to provide an epitaxial stacking structure for TEGFET applications that has a GaXIn1xe2x88x92XP (0 less than Xxe2x89xa61) spacer layer with a superior homogeneity in its indium composition. With this structure, it is possible to provide a GaInP high electron mobility transistor with superior homogeneity in its pinch-off voltage and other properties.
In order to achieve these objects, the present invention provides a GaInP epitaxial structure stacked upon a GaAs single-crystal substrate, comprising at least a buffer layer, a GaZIn1xe2x88x92ZAs (0 less than Zxe2x89xa61) channel layer, and a GaYIn1xe2x88x92YP (0 less than Yxe2x89xa61) electron-supply layer provided joined to the channel layer, the GaInP epitaxial stacking structure including a region within the electron-supply layer wherein the gallium composition ratio (Y) decreases from the side of the junction interface with the channel layer toward the opposite side.
The gallium composition ratio of the aforementioned electron-supply layer is Yxe2x89xa70.51xc2x10.01.
In addition, the gallium composition ratio of the aforementioned electron-supply layer at the junction interface with the channel layer is Yxe2x89xa70.70.
Moreover, the gallium composition ratio of the aforementioned electron-supply layer at the junction interface with the channel layer is Y=1.0.
Furthermore, at the junction interface between the aforementioned electron-supply layer and the channel layer, there is a region with a thickness in the range 1-20 nanometers wherein the gallium composition ratio is constant.
In accordance with another aspect, the invention provides a GaInP epitaxial structure upon a GaAs single-crystal substrate, comprising at least a buffer layer, a GaZIn1xe2x88x92ZAs (0 less than Zxe2x89xa61) channel layer, a GaXIn1xe2x88x92XP (0 less than Xxe2x89xa61) spacer layer, and a GaYIn1xe2x88x92YP (0 less than Yxe2x89xa61) electron-supply layer, wherein the channel layer, spacer layer, and electron-supply layer join each other in this order, and the GaInP epitaxial stacking structure includes a region within the spacer layer wherein the gallium composition ratio (X) decreases from the side of the junction interface with the channel layer toward the side of the electron-supply layer.
The gallium composition ratio of the aforementioned electron-supply layer is Y=0.51xc2x10.01.
In addition, the gallium composition ratio of the aforementioned spacer layer at the junction interface with the channel layer is Xxe2x89xa70.70.
Moreover, the gallium composition ratio of the aforementioned spacer layer at the junction interface with the channel layer is X=1.0.
Furthermore, the gallium composition ratio of the aforementioned spacer layer at the junction interface with the channel layer is X=0.51xc2x10.01.
In addition, a boron-doped n-type layer constitutes the aforementioned spacer layer.
Furthermore, the aforementioned buffer layer consists of a periodic structure of a plurality of AlLGa1xe2x88x92LAs (0 less than Lxe2x89xa61) layers with different aluminum composition ratios (L) vapor-deposited using an organic methyl compound of aluminum or gallium as its starting material, having an AlMGa1xe2x88x92MAs (0 less than Mxe2x89xa61) layer vapor-deposited or the periodic structure using an organic ethyl compound of aluminum or gallium as its starting material.
In addition, the relationship 0.9xe2x89xa6axe2x89xa61.0 holds true for the compensation ratios (K) (K=Na/Nd (if Naxe2x89xa7Nd) and K=Nd/Na (if Nd less than Na); Na: acceptor density of the constituent layer, Nd: donor density of the constituent layer) of the constituent layers of the periodic structure.
The aforementioned periodic structure consists of an AlLGa1xe2x88x92LAs (0xe2x89xa6Lxe2x89xa61) layer and a p-type GaAs layer, and the carrier density of each constituent layer is 1xc3x971015 cmxe2x88x923 or less.
In addition, the aforementioned AlMGa1xe2x88x92MAs layer is touching the channel layer.
Moreover, the aforementioned AlMGa1xe2x88x92MAs layer has a carrier density of 5xc3x971015 cmxe2x88x923 or less, thickness of 100 nm or less and consists of an n-type layer,
Furthermore, the thickness of the aforementioned AlMGa1xe2x88x92MAs layer is less than the thickness of the constituent layers of the periodic structure.
In addition, the aluminum composition ratio (M) of the aforementioned AlMGa1xe2x88x92MAs layer is less than the aluminum composition ratio (L) of the AlLGa1xe2x88x92LAs layers which constitute the periodic structure.
Moreover, the aforementioned buffer layer comprises an AlLGa1xe2x88x92LAs (0xe2x89xa6Lxe2x89xa61) layer vapor-deposited using a trimethyl compound of a Group III element as its starting material, a GaAs layer vapor-deposited using triethyl gallium as the starting material for gallium is disposed between the buffer layer and channel layer, the channel layer has a conduction type of n-type, the spacer layer and electron-supply layer are n-type layers vapor-deposited using trimethyl gallium as the starting material for gallium, the homogeneity in the indium composition ratio within each of the spacer layer and electron-supply layer is xc2x12% or less, and the spacer layer and electron-supply layer are touching each other.
In addition, the surface roughness (haze) after formation of the aforementioned channel layer is 60 ppm or less, and the channel layer touches a GaAs layer vapor-deposited using triethyl gallium as the starting material for gallium.
Furthermore, the aforementioned spacer layer and channel layer touch each other, and the surface roughness (haze) after formation of the spacer layer is 100 ppm or less.
In addition, the surface roughness (haze) after formation of the electron-supply layer is 200 ppm or less.
In accordance with another aspect, the present invention provides a method of fabricating a GaInP epitaxial stacking structure comprising: a step wherein the buffer layer is vapor-deposited using an organic methyl compound of aluminum or gallium as its starting material, a step wherein the AlGaAs layer is vapor-deposited using an organic ethyl compound of aluminum or gallium as its starting material in contact with the periodic structure, and a step wherein the channel layer and electron-supply layer are formed by means of a chemical vapor deposition method using cyclopentadienyl indium which has a bond valence of monovalent as the starting material for indium.
In accordance with another embodiment, the present invention provides a method of fabricating a GaInP epitaxial stacking structure comprising: a step wherein the buffer layer is vapor-deposited using an organic methyl compound of aluminum or gallium as its starting material, a step wherein the AlMGa1xe2x88x92MAs (0xe2x89xa6Mxe2x89xa61) layer is vapor-deposited using an organic ethyl compound of aluminum or gallium as its starting material in contact with the periodic structure, and a step wherein the channel layer, spacer layer and electron-supply layer are formed by means of a chemical vapor deposition method using cyclopentadienyl indium which has a bond valence of monovalent as the starting material for indium.
Moreover, the present invention also comprises a field effect transistor fabricated using the aforementioned GaInP epitaxial stacking structure.
As described above, the present invention constitutes the electron-supply layer as a GaYIn1xe2x88x92YP layer with a gradient in the composition such that the gallium composition ratio decreases in the direction of increasing layer thickness from the channel layer toward the contact layer, so a two-dimensional electron gas efficiently accumulates in the interior of the channel layer, and a high electron mobility is manifested, so a GaInP epitaxial stacking structure with a superior homogeneity in the transconductance and pinch-off voltage can be provided.
In addition, as described above, the present invention constitutes the spacer layer as a GaXIn1xe2x88x92XP layer with a gradient in the composition such that the gallium composition ratio decreases in the direction of increasing layer thickness from the channel layer toward the contact layer, so a two-dimensional electron gas efficiently accumulates in the interior of the channel layer, and a high electron mobility is manifested, so a GaInP epitaxial stacking structure with a superior homogeneity in the transconductance and pinch-off voltage can be provided.
Moreover, as described above, the present invention constitutes the superlattice periodic structure constituting one part of the buffer layer with a periodic alternating layer structure of AlLGa1xe2x88x92LAs layers vapor-deposited using an organic methyl compound as its starting material and with a stipulated compensation ratio, so a GaInP epitaxial stacking structure with a low leakage current can be provided.
Furthermore, the constitution is such that an indium-containing Group III-V compound is provided with a GaAs thin-film layer vapor-deposited from triethyl gallium as its starting material, so a GaZIn1xe2x88x92ZAs channel layer, a GaXIn1xe2x88x92XP spacer layer, and an electron-supply layer with superior homogeneity in indium composition can be formed, and therefore, a GaInP epitaxial stacking structure with a superior homogeneity in the transconductance and pinch-off voltage can be provided.
The above and other objects and features of the invention will become apparent from the following description made with reference to the drawings.